1. Field of the Invention
The present invention relates generally to electron beam systems. The present invention more particularly relates to voltage contrast detection of defective structures using scanning electron microscopes.
2. Description of the Background Art
The defects detected by a state-of-the art scanning electron microscope (SEM) inspection system may be divided roughly into two categories: local contrast (physical) defects; and voltage contrast defects.
For local contrast defects or physical defects, contrast formation is directly related to the emission of secondary and/or backscattering electrons from the defect in question. These defects are generally located at the positions where the contrast appears (in x, y, and z directions). Local contrast defects generally manifest themselves due to its chemistry (i.e. atomic number Z contrast) and/or geometric properties (i.e. topographic contrast).
For voltage contrast defects, contrast formation is due to the fact that the probability that secondary electrons escape from a specimen may be modulated substantially by the surface potential. A voltage contrast defect may be regarded as electrically active. The defect is electrically active in that it is capable of enabling electron bombardment to alter the electrostatic potential of the target in which the defect exists. Voltage contrast may be “long range” in that the contrast may appear at positions which are relatively far away from the actual physical location of the defect. Voltage contrast does not require direct interaction between the primary electron beam and the defect in question. This is because voltage contrast formation needs no involvement of electrons emitted directly from the defect.
The long-range character of voltage contrast defect detection plays a role in at least three aspects. First, the visibility of a voltage contrast defect is related to the defect's capability to help change the surface potential. The physical size of the defect becomes of less importance. As a consequence, defect detection with voltage contrast may be enhanced by system features other than higher resolution. Second, detection of subsurface defects is enabled by the spatial displacement of voltage contrast in the z-direction (the direction normal to the surface). This capability may be especially useful for a multi-layer structure, such as, for example, modern interconnect technology, where a substantial amount of defects are often formed in underlying layers. Third, the “long range” character of voltage contrast (i.e. the fact that direct interaction between the primary electron beam and the defect is not needed) makes it possible to inspect a device by scanning only a small percentage of the total area of interest. As a result of this aspect, voltage contrast defect detection may be used to accelerate (reduce the time needed to perform) inspection of a specimen. The specimen may be, for example, a semiconductor wafer with memory arrays or other circuitry thereon.
Hence, voltage contrast has capabilities and features which make it a useful and advantageous technique for detecting defects. Nevertheless, useful as it is, voltage contrast is often a complicated phenomenon to harness effectively.
A first complication or problem is that voltage contrast is a combined result of the escape probability of secondary electrons and the collection efficiency of the detector. Each of these two factors depend on surface potential distribution in its own way. This complicated dependency is exacerbated by the fact that the electric field on the specimen surface is rarely uniform and is to a large extent affected by the apparatus configuration. The transverse component of the electric field, which is usually non-zero, may often cause secondary electrons to accelerate in the direction parallel to the specimen surface so as to evade the detector altogether.
A second complication or problem is that additional failure analysis is typically needed to correlate a voltage contrast defect to an actual physical defect. For example, a missing via in copper interconnection technology may cause voltage contrast and so be detectable. However, it typically takes tedious failure analysis to confirm this defect because it lies beneath the surface.
At the transistor level, electrical failure may have many different mechanisms and current leakage may go through many different routes. Each leakage path may potentially generate the voltage contrast observed. Voltage contrast imaging typically results in a “mosaic” combining effects from various leakages. The defect of interest (DOI) or the leakage path of interest is typically either overwhelmed by the “background noise” effects from the defect population or may be undetectable.
Thus, applicants respectfully submit that it is desirable to provide a method to isolate the leakage path of interest while suppressing or filtering out the background noise from other defects. An advantageous goal of such a method is to make the defect detection more useful and reliable.